Embossed core wrap for electrical cables

ABSTRACT

An electrical cable having a conductor core and a protective covering therefor with an embossed polyester film interposed between the periphery of the conductor core and the protective covering. The embossed film provides an essentially liquid-tight seal about the conductor core and affords mechanical protection, thermal insulation, dielectric protection and electrical stability for such core.

United States Patent Willmore 1 Oct. 31, 1972 [54] EMBOSSED CORE WRAP FOR 463,107 10/1891 Degenhardt ..174/28 X ELECTRICAL CABLES 3,575,781 4/1971 Pezely ..161/ 131 3,332,138 7/1967 Garner ..'.l74/l07 X [72] Invent g chaste" 3,509,269 4/1970 Elliott ..174/107 x y 3,304,214 2/1967 Alm ..174/107 X [73] Assignee: FMC Corporation, Philadelphia, Pa. 3,294,387 12/1966 Chavannes ..161/ 127 X [22] Filed: May 1971 Primary Examiner-Bernard A. Gilheany [21] Appl. No; 142,211 Assistant Examiner-A. T. Grimley AttorneyThomas R. OMalley and Eugene G. Horsk [52] U.S. Cl ..174/107, 161/131, 161/DIG. 3, a y v 174/23 R, 174/102 D 57 T T [51] Int. Cl. ..H0lb 7/24 [58] Field of s 174/2 24, 25 C 26 R, 102 R, An electrical cable having a conductor core and a prol74/102 D 116 107 36 23 161/125 tective covering therefor yvith'an embossed polyester 127 128 156/54 film interposed between the periphery of the conductor core and the protective covering. The embossed I film provides an essentially liquid-tight sea] about the conductor core and affords mechanical protection, [56] References Cited thermal insulation, dielectric protection and electrical UNITED STATES PATENTS I stability for such core. 3,244,799 4/ 1966 Roberts ..174/ 107 5 Claims, 6 Drawing Figures EMBOSSED CORE WRAP FOR ELECTRICAL CABLES The present invention is directed to anelectrical cable having an improved protective barrier for the cable is formed of single thickness (non-laminated) polyester film, with the corrugations thereof being uniform and extending longitudinally thereof, and may be either wound spirally about the conductor core or applied longitudinally thereto with their longitudinal edges overlapped.

In electrical cables as described in the cited patent, the corrugated tape provides air spaces in the area between the conductor core and the surrounding metallic shield which serve to insulate the core against the transfer of heat applied to the metalliclshield during jacketing of the cable with the molten thermoplastic material. These air spaces alsopneumatically cushion the conductor core against mechanical forces applied to the cable and which might otherwise damage the core insulation. ,Q

It is well known in the art that polyester film is well suited for use as a protective barrier in electrical cables since such film does not absorb moisture, has a low dielectric constant, low dissipation factor i and high dielectric strength, and retains these desirable characteristics during the operating life of electrical cables. in the tape of polyester film described in the above-noted patent, the longitudinally extending corrugations do enhance certain of the characteristics. which the polyester film itself possesses. However, the presence of these longitudinally extending corrugations do not remedy and may perhaps introduce or amplify problems which are commonly encountered in making of electrical cables having an electrical insulating jell contained between the wires of the conductor core and barrier tape. i

More particularly, in the manufacture of conventional jell-filled electrical cables, wires which are to form the cable core are coated with a relatively heavy layer of an electrically insulating jell which may be, for example, a blend of petrolatum and polyethylene. Generally, this coating operation is achieved by passing the core wires through a bath of molten jell. Once coated, the wires are squeezed tightly together, wrapped with a barrier tape, and covered with a metallic sheath and a jacket of thermoplastic material. Squeezing of the coated core wires together, prior to the application of the barrier tape, serves to remove excess jell and causes all voids about the core wires to be filled. With the removal of such voids all air is excluded from the core jell covering and the possibility of moisture entering into such core, in the event the jacket is subsequently damaged, is eliminated.

Before the outer protective metallic sheath and jacket can be applied, leakage of the jell from between the overlapping edges of the barrier tape is often experienced. Such leakage, of course, involves a waste of materials, creates processing problems and, most important, provides electrical cables with variable and unreliable characteristics.

Significantly, when corrugated barrier tapes, as heretofore described,'are employed in the manufacture of jell-filled electrical cables, there is a pronounced tendency for the electrical insulating jell to escape. It appears that this effect is due to the longitudinal corrugations in the barrier tape serving as flow channels for the jell, permitting such jell to travel freely toward overlapping edges of the tape at which leakage may occur. Accordingly, a primary object of this invention is to provide an improved protective barrier for a cond uctor 'core of an electrical cable.

Another object of this invention is the provision of an electrical cable having a conductor core surrounded with a protective barrier formed of embossed polymeric film, and particularly polyethylene terephthalate film, which affords mechanical protection, thermal in- I sulation, dielectric protection, and electrical stability for such core.

Still another object of the present invention is to provide a protective barrier which is adapted to form an essentially liquid-tight seal about a conductor core of an electrical cable.

A further object is to provide an electrical cable having an electrical insulating jell contained between a conductor core and a protective barrier formed with closely packed embossments which obstruct or retard jell flow. 1

A still further object is to provide, for use in electrical cables, a protective barrier having embossments which are. adapted to interlock with each other when overlapped and thereby form an essentially fluid-tight joint therebetween.

These and other objects of the invention are achieved by the use of an embossed polymeric film as a protective barrier between a conductor core and an outer protective coveringof an electrical cable. This protective barrier is preferably formed of polyethylene terephthalate film and is hereafter described as such. However, films formed of other polymeric materials may be employed, providing such films do not absorb moisture, have a low dielectric constant, low dissipation factor, high dielectric strength and are capable of being embossed and permanently retaining such embossments.

The embossed polyester film forming the protective barrier of the present invention is applied as a tape or strip, either longitudinally or spirally about the conductor core, with its longitudinal edges in overlapping relationship. Once applied, the embossments on such tape form air spaces between the conductor core and an overlying metallic sheath which, together with a jacket of thermoplastic material, constitutes the protective outer covering of the cable. These contained air spaces serve as a thermal insulation and pneumatically cushion the conductor core from external forces.

The polyester film and embossments thereon together provide mechanical protection, thermal insulation, dielectric protection and electrical stability for the conductor core which is many times greater than that which is afforded by a flat polyester film of like thickness. With electrical cables filled with an electrical insulating jell, the embossments on the tape of polyester film serve also to restrict or retard the flow of the jell along the conductor core. More important, overlapping edge portions of such embossed tape, whether applied longitudinally or spirally of the conductor core, interlock with each other. That is, embossments projecting from the surface of one of such overlapping edge portions mate and grip firmly with depreswith each other. More particularly, the embossments are aligned in substantially parallel rows, with the em bossments in each of such'rows being in staggered relationship with the embossments in the rows directly adjacent thereto whereby only circuitous flow paths are present along opposite sides of such-film. Preferably, the embossments are arranged in almost abutting relationship. The perimeters of the embossments may be circular or may be defined by straight and/or arcuate lines. Desirably, the perimeters of the embossments are of polygonal contour, as for example of hexagonal configuration, to facilitate a close-packed staggered arrangement thereof, and are separated from each other only by linear portions of the film.

In electrical cables, the embossed polyester film of the present invention affords the above-described advantages regardless of whether its embossed or recessed side is in facing relationship with theconductor core.

For a greater understanding of this invention, reference is made to the following detailed description and accompanying drawing inwhich FIG. 1 is a side view of an electrical cable which incorporates the protective barrier of the present invention, with portions of such cable being broken away to illustrate various elements thereof;

FIG. 2 is a transverse section through the electrical cable taken substantially along the line lI-Il of FIG. 1;

' FIG. 3 is a plan view of a portion of the protective barrier employed in the cable shown in FIG. 1;

FIG. 4 is a vertical section taken substantially along the line IV-IV of FIG. 3;

I FIG. 5 is a vertical section taken substantially along the line V-V of FIG. 3, and

FIG. 6 is a section, similar to that shown in FIG. 3, taken through overlapping edge portions of the protective barrier.

With reference to FIGS. 1 and 2 of the drawing, the electrical cable there illustrated is indicated generally by the character 11 and includesa conductor core 13 composed of a plurality of individually insulated wires, a protective barrier 15, an electrical insulating jell 17 in between the wires of the core 13 and the protective barrier 15, a metallic sheath 19 over the protective barrier l5, and a jacket 21 of thermoplastic material. The cable 11 is assembled in the conventional manner with the core wires being first passed through a bath of molten jell l7, and then squeezed together to force out excess jell and to fill all voids in the remaining jell. Protec- .roll.

tive barrier 15, in the form of a tape or strip, is then applied longitudinally of the conductor core 13 with its longitudinal edges in overlapping relationship. After which, the metallic sheath 19 is provided over the wrapped core and I is covered with a molten thermoplastic material to form the jacket 21 thereon.

Illustrated greater detail in FIGS. 3-5 is the protective barrier 15 which is formed of a film of polyethylene terephthalate having a normal flat thickness of perhaps 0.002 inches. Polyethylene terephthalate films of greater thickness can be employed providing they are capable of being embossed and permanently retain the embossment or deformations set therein. Embossing of the polyethylene terephthalate film may be achieved in any convenient manner, asfor example byheating such film to a temperature below its softening point, followed by nipping such film, under relatively high pressure, between a cool, engraved metal roll and a rubber covered back-up The pattern-of the embossments 23 formed in the polyethylene terephthalate film are of like size and configuration and are preferably in a closely-packed array so that only linear portions of the original film appear between adjacent embossments. The perimeters of the individual embossments 23 may be of any desired contour and desirably are of polygonal configuration, as for example of a hexagonal pattern asillustrated, to facilitatea close packing thereof.

More particularly, in the protective barrier 15 shown in FIGS. 3-5, the hexagonal embossments 23 are arranged in substantially parallel rows, as indicated by the lines 25, with the embossments in the directly adjacent rowsbeing in staggered relationship with each other. With this arrangement, only linear portions 27 of the original film remain between adjacent embossments 23. The lengths of these individual linear portions 27 will, of course, depend upon the particular configuration and size of the embossments themselves. The staggered array of the closely spaced embossments provides very restricted circuitous paths along which flow of the jell 17 during cable assembly is difficult and continually retarded. More important, as illustrated in FIG. 6 overlapping edge portionsof the barrier 15 interlock with each other, that is, embossments 23 projecting from the surface of one of such overlapping edge portions mate and are gripped firmly with depressions 29 in the surface of the other of such overlapping edge portions. As a result of this firm interlocking, these overlapping edge portions of the protective barrier 15 together provide a compact and snug or essentially liquid-tight joint.

It is to be understood that changes and variations may be made without departing from the spirit and scope of the invention as defined in the appended claims.

I claim: I

1. In an electrical cable of the type embodying a conductor core, a protective barrier for said core, which barrier does not absorb moisture and has a low dielec-.

formed of a flexible polymeric film enveloped about said conductor core with. its longitudinal edge portions in overlapping relationship, said flexible polymeric film having rows of permanently pre-set, substantially like, individual embossments projecting from one surface thereof, said individual embossments being closely packed and in almost abutting relationship with each other, and permanently pre-set, substantially like, individual depressions in the other surface thereof, with said depressions being aligned with said embossments, the rows of embossments in said polymeric film being arranged in substantially parallel relationship, the material and surface contour of said polymeric film serving to thermally insulate said core, particularly during the application of the molten plastic material to the metallic shield, and providing mechanical protection, dielectric protection and electrical stability for said core.

2. In a cable as defined in claim 1 wherein the embossments in each of the rows of embossments are in staggered relationship with the embossments in the rows directly adjacent thereto.

3. In a cable as defined in claim Zwherein the embossments projecting from one surface of said polymeric film interlock with depressions in the other surface of said polymeric film along the overlapping portions thereof to provide an essentially liquid-tight joint therebetween.

4. In a cable as defined in claim 3 further including an electrical insulating jell in between said conductor core and polymeric film.

5. In a cable as defined in claim 2 wherein the perimeters of said embossments are of polygonal contour and the portions of the film between adjacent embossments are linear. 

1. In an electrical cable of the type embodying a conductor core, a protective barrier for said core, which barrier does not absorb moisture and has a low dielectric constant low dissipation factor and a high dielectric strength, a metallic shield disposed over said protective barrier, and a continuous covering of plastic material solidified on said metallic shield from a molten condition, the improvement comprising a protective barrier formed of a flexible polymeric film enveloped about said conductor core with its longitudinal edge portions in overlapping relationship, said flexible polymeric film having rows of permanently pre-set, substantially like, individual embossments projecting from one surface thereof, said individual embossments being closely packed and in almost abutting relationship with each other, and permanently pre-set, substantially like, individual depressions in the other surface thereof, with said depressions being aligned with said embossments, the rows of embossments in said polymeric film being arranged in substantially parallel relationship, the material and surface contour of said polymeric film serving to thermally insulate said core, particularly during the application of the molten plastic material to the metallic shield, and providing mechanical protection, dielectric protection and electrical stability for said core.
 2. In a cable as defined in claim 1 wherein the embossments in each of the rows of embossments are in staggered relationship with the embossments in the rows directly adjacent thereto.
 3. In a cable as defined in claim 2 wherein the embossments projecting from one surface of said polymeric film interlock with depressions in the other surface of said polymeric film along the overlapping portions thereof to provide an essentially liquid-tight joint therebetween.
 4. In a cable as defined in claim 3 further including an electrical insulating jell in between said conductor core and polymeric film.
 5. In a cable as defined in claim 2 wherein the perimeters of said embossments are of polygonal contour and the portions of the film between adjacent embossments are linear. 